Many process control valves are operated using actuators. Actuators automate control valves by supplying force and motion to open or close a valve. Rotary valves such as butterfly valves, ball valves, disk valves, etc. typically have a valve shaft that drives a flow control member between an open position and a closed position. In particular, the flow control member of a rotary valve is typically rotated within a valve body passage to vary the flow of fluid therethrough. A rotary actuator is an actuator that that may be used to supply rotating force and motion to rotate the valve shaft of a rotary valve. Rotary actuators typically include an actuator stem that couples to the valve shaft.
A common type of rotary actuator is a scotch yoke rotary actuator, which converts linear motion to a quarter turn rotation motion. A scotch yoke rotary actuator includes a push rod that moves back-and-forth, via a linear actuator, along a rotatable shaft or stem that is offset from and perpendicular to the rod. The push rod is coupled to the rotatable shaft via a yoke or arm that converts linear motion to rotary movement of the shaft. In particular, the rod is coupled to the yoke by a pin that passes through a slot or opening in the yoke. As the rod moves back-and-forth, the pin causes the yoke to rotate (as the pin slides in the slot), thereby rotating the stem about its axis. Although effective for opening and closing a valve, scotch yoke rotary actuators have inefficiencies due to yoke friction and side forces. As mentioned above, as the rod translates, the pin slides along the inner surfaces of the slot. This sliding interaction generates friction, which decreases the efficiency of the rotary actuator. Additionally, due to sliding forces, not all of the linear force from the push rod is converted into rotational force. Instead, side forces are generated in the yoke, which are not converted into rotational force and, thus, also decreases the efficiency of the actuator.